The Paramyxovirus family contains numerous human and animal pathogens of global concern. Nipah (NiV) and Hendra (HeV) viruses of the Henipavirus (HNV) genus are the most virulent paramyxoviruses that infect humans. Zoonotic transmission of NiV and HeV from their natural fruit bat reservoirs to humans can result in mortality rates in excess of 90%. The recent discovery of novel clades of HNV across the globe highlights the possible global emergence of these zoonotic viruses. A better understanding of the basic pathobiology of highly pathogenic viruses is critical for developing therapeutic options. The Paramyxovirus matrix (M) protein is critical to viral assembly and budding. Although paramyxoviruses replicate entirely in the cytoplasm, we found that early during infection, NiV-M protein is first targeted to the nucleus prior to ubiquitination and subsequent localization to the plasma membrane. This ubiquitin-regulated nuclear-cytoplasmic trafficking of M is critical for its ability to mediate viral assembly and budding, and is conserved across viruses from several Paramyxovirus genera. This application is spurred by a confluence of further preliminary data suggesting that NiV-M is SUMOylated, and that the SUMOylation state of the cell affects NiV-M's function. Our overall goal is to determine the role of SUMO and ubiquitin modifications in matrix trafficking and function. Our objective is to understand how cognate SUMO and ubiquitin proteasome pathway components coordinately regulate the complex intracellular trafficking behavior of HNV-M proteins, and to uncover the functional role(s) that these post-translational modifications play in the virus replicative life cycle. Our driving hypothesis is that specific SUMO and ubiquitin proteasome pathway components coordinate the spatial-temporal control of the matrix protein's nuclear sojourn, which enables the proper membrane targeting of matrix that is critical for virus assembly and budding. Understanding the basic cell biology of HNV-M might reveal host-pathogen interactions that could be targeted for therapeutic interventions. To interrogate our driving hypothesis, we propose the following Specific Aims: AIM 1: Determine which lysines in HNV-M are modified. AIM 2: Interrogate the consequences of HNV-M interactions with E3 Ub-ligases. AIM 3: Evaluate the role of SUMOylation and Nup358/RanBP2 (a SUMO E3 ligase) in the nuclear trafficking and function of HNV-M proteins. AIM 4: Define the functional roles for matrix post-translational modifications in relation to viral replication and pathogenic fitness.